I've lurked this site for info a few times.
Just ran across this thread and after I read through it I am disappointed in how many ignorant people are posting. I am personally a software developer and would like to clear up a few misconceptions that I have read here.
This is strictly because I don't like ignorance.
First of all, an accelerometer does not measure how fast you are moving with respect to gps. An accelerometer is a sensitive device that measures the ACCELERATION of your phone by acting as an intricate, small scale. Accelerometers in phones are what would tell if your phone is on its side or its back as well as many other things. Do you really think that to switch between portrait and landscape mode, your phone has to consult a gps system?
In this case the accelerometer is just being used to find out which way gravity is pointing with respect to your phone.
When you put your phone on the bag it gives it a somewhat free-moving service to lay on. You put the coin on the phone and the accelerometer will measure how much the phone's pitch has changed compared to the estimated weight of a coin.
Now when you put weed on after calibrating, the idea is that you should be able to create a relation between the new pitch and the pitch of the phone with just a coin on it. Theoretically, this relationship can be used to calculate the weight of the added weed.
You people saying that it is an impossible concept are either ignorant to the android operating system or the function of an accelerometer.
Now as to why it actually DOESN'T work, you will need to understand some concepts of physics.
When the phone is on an inflated bag and you place something on it's screen, the weight of that object is effectively applying a TORQUE to the system.
Torque is proportional to
the force exerted on an object
the distance between the that force and the pivot point (this case the center of the phone)
and the sine function of the direction of the force and the object
Because of it's dependency on the distance from the pivot point, if you don't keep the spot that you place the weed on EXACTLY where the coin was in calibration, it will cause a problem.
Because the screen of a phone is smooth and almost frictionless, the weed will slide, changing the distance which will cause problems.
Because the phone is a non-uniform 2d service, the pitch will not act the same as more weight is added.
Lastly, because a non-uniform bag filled with air doesn't compress uniformly for all weights there will be problems.
The app works as a proof of concept. This is possible to accomplish, but in the current design they use, it is far too inaccurate and unreliable to be used in any situation involving money.
Anyways, I just posted all of this because the original poster should not be accused of trolling and I dislike ignorance.
Knowledge is power